VISION

First of all, this project aims to contribute to the understanding of the fundamental physical mechanisms of the photosensitivity of polymers and to the improvement of the polymer fiber grating technology. Then, it aims to provide high sensitivity miniaturized sensors, as a key component of new intelligent materials and devices able to improve performance in sports and quality of life.

SCIENTIFIC TOPICS

Strain, stress and temperature are directly encoded by the FBG wavelength, while other effects (e.g. vibration, shape, pressure) can be derived by the strain induced on the fiber. Polymer optical fibers (POFs) are highly elastic as compared to silica fibers and can host high sensitivity FBG sensors, as very recently demonstrated. POFs have advantages of great flexibility and low material cost, and allow for the doping of a wide range of organic materials. The photosensitivity in polymer optical fibers and its applications in fiber Bragg grating sensors for the realization of smart systems will be investigated.

APPLICATIONS

The ultimate aim is the integration of optical fiber sensors into materials for i) the measurement of parameter fields like temperature or strain inside the material, ii) the comparison with model calculations and state of the art measurements to validate models in use, iii) the realization of smart structures that contain different types of actuators and sensors - the smart structure. For example, wearable-instrumented devices containing polymer optical fiber sensors would be capable of recording biomechanical variables and would be of interest to several fields of application, from multimedia to physical rehabilitation, from sports to artistic fields.

RESULTS

The photosensitivity of a SM-POF was investigated by directly writing FBGs into the fiber using a phase mask setup and an Excimer laser emitting at 308 nm. Most of the written gratings showed a loss of refractive index modification within a couple of days. Some of the gratings showed no significant index modification change over several months, and where suitable for extended characterization. The maximum reflectivity achieved with a 5-mm-long FBG was 70% corresponding to an index change of ∆n_ac=1.7x10^-4.

Figure 1 shows POF-FBG spectra for different temperatures. With increasing temperature a shift to smaller wavelengths. Figure 2 shows the temperature dependence of POF-FBG. The sensitivity is -136 ± 5 pm/°C. In contrast to standard silica fibers (SMF-28 10 pm/°C at 1550 nm) this temperature dependence of the Bragg wavelength is negative and more than one order of magnitude larger. It therefore may enable the realization of temperature sensors with increased sensitivity using POF-FBGs as compared to silica fiber FBGs. However, the temperature range in which the POF-FBG can be used is smaller than for silica fibers due to the glass transition temperature of PMMA and PS being around 100 °C and the index decay that is accelerated by temperature: Already at temperatures of 60 °C, an index decay of the written FBG could be observed.

FBG spectra at different temperatures.

Bragg wavelength versus Temperature.